1. Technical Field
The present invention relates to a printer that performs color printing by superimposing ink colors while reciprocating and conveying a recording sheet by sheet feed means provided on the downstream and upstream sides in a conveying direction of a recording sheet, and a method of adjusting the conveying distance of a recording sheet.
2. Related Art
A printer, which performs color printing by superimposing ink colors through thermal transfer while reciprocating and conveying a recording sheet, has been used in the past (see Japanese Unexamined Patent Application Publication No. 2008-114415). In general, the printer disclosed in Japanese Unexamined Patent Application Publication No. 2008-114415 includes first sheet feed means that is provided on the upstream side in a conveying direction of a recording sheet and second sheet feed means that is provided on the downstream side. The first and second sheet feed means includes first and second sheet feed rollers and first and second pressing rollers (not shown), respectively. The first and second sheet feed rollers can be rotationally driven. The first and second pressing rollers come into press contact with the first and second sheet feed rollers, respectively, and can be rotationally driven. A recording sheet is nipped between the first and second sheet feed rollers and the first and second pressing rollers of the respective sheet feed means, and is conveyed in the normal or reverse direction by the normal or reverse rotation of the sheet feed rollers.
Further, printing means for performing desired printing on a recording sheet is disposed between the first and second sheet feed means. The printing means includes a platen roller that may be rotationally driven by a driving force transmitted from a drive motor, and a thermal head that may come into contact with and be separated from the platen roller. Furthermore, an ink ribbon on which a plurality of ink layers is continuously formed is led between the thermal head and a recording sheet, the thermal head is moved down so that the ink ribbon and the recording sheet are nipped between the platen roller and the thermal head, and ink of the ink ribbon is thermally transferred to the recording sheet by selectively supplying current to heating elements of the thermal head.
Further, gears are mounted on the end portions of the platen roller and the sheet feed rollers, respectively (hereinafter, referred to as a platen gear, a first sheet feed gear, and a second sheet feed gear, respectively). Intermediate gears are provided at an intermediate position between the platen gear and the first sheet feed gear and at an intermediate position between the platen gear and the second sheet feed gear, respectively. Accordingly, a driving force, which is transmitted to the platen gear from the drive motor, is branched and transmitted to the respective sheet feed rollers, so that the respective sheet feed rollers are rotationally driven.
The intermediate gears are supported by gear support shafts, respectively. Each of the gear support shafts is disposed so as to adjust backlash between each of the intermediate gears and the sheet feed gears by changing the center distances between each of the intermediate gears and the sheet feed gears and so as to adjust timing where the driving force of the drive motor transmitted to the platen gear is transmitted to the respective sheet feed gears through the respective intermediate gears. That is, it may be possible to adjust timing where the respective sheet feed rollers start to be rotated after the start of the drive motor (hereinafter, referred to as start timing).
Meanwhile, since the printer disclosed in Japanese Unexamined Patent Application Publication No. 2008-114415 controls a conveying distance of a recording sheet by the number of steps of the drive motor, a difference between the conveying distance of a recording sheet that is conveyed in a conveying direction and the conveying distance of a recording sheet that is fed back in a reverse conveying direction has been generated by the reason such as the time lags of the start timing of the first and second sheet feed rollers. In this case, if images of ink colors are sequentially thermally transferred to a recording sheet while the recording sheet is reciprocated and conveyed, there is a problem in that printing failure such as positional deviation of the images of the respective ink colors, that is, so-called color deviation is generated in an image actually printed on the recording sheet.
Further, in the above-mentioned printer, the adjustment of the position of the gear support shaft of each of the intermediate gears, that is, the adjustment of a center distance between each intermediate gear and each sheet feed gear is performed by mounting a plurality of blocks, which has a pair of mounting holes formed to be separated from each other by a desired center distance, on the gear support shafts and the support shafts of the sheet feed rollers and the platen roller, respectively. In this adjustment, there are problems in that a plurality of blocks, which has a pair of mounting holes formed to be separated from each other by a predetermined center distance, needs to be previously produced and it is difficult to perform an adjusting operation. Furthermore, since the center distance between the respective gear support shafts is determined by the positions of the pair of mounting holes, it may not be possible to severely adjust the center distance. Moreover, as a result of the above-mentioned adjustment, if backlash between the first idler gear and the first sheet feed gear and backlash between the second idler gear and the second sheet feed gear are excessively small, so-called jitter has been generated on a printed image.
These and other drawbacks exist.